A 3-level inverter produces a 3-level voltage, e.g., a high voltage, a medium voltage, and a lower voltage, by subdividing a DC power voltage (stringing voltage) into two DC voltages by way of series-connected capacitors, and selectively conducts these three leveled voltages to an inverter output terminal by turning ON/OFF the switching elements of the major circuit. The 3-level inverter has the following features.
That is, since the step number of the output voltage pulse is increased, the virtual switching frequency is increased and then an output with less distortion is obtained. Since the voltage applied to the element is reduced to approximately 1/2, as compared with that of the 2-level inverter, switching elements with relatively low-withstanding voltages may be utilized. Since the voltages applied to the switching elements are lowered, loss occurring in the switching elements may be reduced.
As the generating/controlling method for the output voltage pulse of the above-described 3-level inverter, the following methods have been proposed:
(1) "NEW DEVELOPMENTS OF 3-LEVEL PWM STRATEGIES" (EPE' 89 Record, 1989), page 412. In FIG. 1, there are shown a so-called "dipolar modulation" (the output voltage is produced by alternately outputting the positive/negative pulses via the zero voltage within a half period of the output voltage), a so-termed "unipolar modulation" (the output voltage is produced by outputting the pulse with a single polarity within a half period of the output voltage), and also a method for switching the above-described dipolar modulation and unipolar modulation. PA1 (2) "PWM Systems in Power Converters: an Extension of the "Subharmonic" Method" (IEEE Transaction on Industrial Electronics and Control Instrumentation, vol. IECI-28, No. 4, November 1981), page 316. In FIG. 2(b), such a modulation method (will be referred to "overmodulation") has been proposed. That is, a half period of the output voltage is constructed of a plurality of pulses each having a single polarity, and the output voltage is produced by reducing the number of pulses in such a manner that slits among the pulses are filled up from the center portion of this half period. PA1 (3) STUDY OF 2 AND 3-LEVEL PRECALCULATED MODULATIONS (EPE' 91 Record, 1991), page 411. In FIG. 16, the output voltage pulse generating/controlling method has been proposed for covering the output voltage from 0 to 100%. PA1 a dipolar modulation mode for producing a series of output pulses such that a half period of a fundamental wave of an output phase voltage of this electric power converter is represented by a pulse series having a zero potential between a positive pulse and a negative pulse; PA1 a unipolar modulation mode for producing a series of output pulses such that a half period of a fundamental wave of an output phase voltage of said electric power converter is represented by a pulse series constructed of a plurality of single polarity pulses; PA1 an overmodulation mode for producing a series of output pulses such that a half period of a fundamental wave of an output phase voltage of the electric power converter is represented by reducing the number of pulses in such a manner that a slit between the pulses is filled up from a center of a pulse train constructed of a plurality of single polarity pulses; PA1 a 1 pulse mode for producing an output pulse such that a half period of a fundamental wave of an output phase voltage of the electric power converter is represented by a single pulse having the same polarity; and PA1 a means for changing the modulation mode among the dipolar modulation mode, unipolar modulation mode, overmodulation mode and 1 pulse mode. PA1 first pulse generating means for generating pulses under an asynchronous condition with a fundamental wave of an output voltage from the electric power converter, thereby outputting a plurality of pulses at a half period of an output phase voltage; and PA1 second pulse generating means for generating at the electric power converter a single pulse having the same polarity as that of the fundamental wave of the voltage outputted from the electric power converter within a half period of the output phase voltage in synchronism with this fundamental wave of the output voltage. PA1 control means for continuously changing a switching frequency of the inverter within all regions that there are plural pulses within a half period of the fundamental wave of an output voltage from the inverter.